Review Article: Reactions of fluorine atoms with silicon, revisited, again

Abstract

Chemical etching of silicon by fluorine atoms in the absence of ion bombardment is reviewed. Controversies on the identity of etching products and reaction probabilities are discussed. Attempts are made to explain the apparent presence of SiF2 as a primary product in many studies, dating back to 1980, but not in others, including those of Harold Winters from as early as 1979. Reported estimates of reaction probabilities (here defined as the probability of removing a Si atom from the substrate per incident F atom) vary by a factor of 2000. When these values, with some corrections and reasonable adjustments, are plotted as a function of F atom flux, most of them fall on a “universal curve” that reveals a large (∼30-fold) decrease in the reaction probability with increasing F flux, from 0.03 at a F flux 1012 cm−2 s−1 to 0.001 at a flux of 1020 cm−2 s−1. These values were extracted from beam experiments with F atoms generated from cracking of F2, including those by Harold Winters, from isotropic etching in plasma experiments (both in-plasma and downstream) with F2, CF4/10%O2, and NF3 feed gases, as well as from molecular dynamics simulations. Reaction coefficients derived from chemical etching rates in SF6 plasmas do not follow this trend, however, suggesting a large enhancement in the F reaction probability (∼20 to 100-fold at F fluxes of 1018–1019 cm−2 s−1), due to the presence of sulfur.